Omnidirectional loudspeakers are often used for acoustical measurements rather than for sound reproduction.
That being said, the Spat model essentially assumes that the loudspeakers are oriented towards the (center of) listening area.
Using omnidirectional speakers, you would increase the interaction with the listening room (i.e. generate reflections)
and this may interfere with the spatialization rendering.

I heard a system which used omni speakers – but not spat~ and it sounded great. It had a less obvious sweet spot which helps in situations where there are many people in the space as they all hear good spatialised audio.

Would have liked to try a system like this with spat~ – but sounds like it would produce unpleasant reflections like you say

Any tips on expanding the sweet spot on a system designed using spat would be welcome.